WERE THE DUCHESS with us today, she might have added, "Or too fat-free."

And she would have gotten plenty of support, especially from some of the medical community, which would have added men and children older than 2 to her domain.

The rule of thumb these days is that not more than 30 percent of a person's calories should come from fat. The Dietary Guidelines for Americans, developed and disseminated by the Department of Agriculture and the Department of Health and Human Services, lay out this level of consumption, along with 15 percent of calories from protein and the remaining 55 percent from carbohydrates.

Many nutritionists and health organizations, taking their lead from the guidelines, espouse these numbers too. But others say that getting even 30 percent of a day's calories from fat is too much.

The Center for Science in the Public Interest, a Washington consumer advocacy group that has pointed out problems in the nation's food supply for the last 20 years, recommends in its May newsletter, "Nutrition Action," that adults get 20 percent of their calories from fat.

More restrictive, several physicians speaking at a recent conference on the relationship of diet to health said a 10 percent fat diet was "optimal" for good health.

The average American, however, has much more than 30 percent fat in his diet. A USDA survey in 1987-88 showed that those surveyed received 37 percent of their calories from fat, 15 percent from protein and 48 percent from carbohydrates.

"A low-fat vegetarian [diet] is optimal for good health," said Dr. Dean Ornish, an assistant clinical professor at the University of California at San Francisco. He has researched reversing heart disease through lifestyle changes and written a book about his work, "Dr. Dean Ornish's Program for Reversing Heart Disease."

The diet that Ornish advocates, especially for people with heart disease, includes no animal products except egg whites, skim milk and other no-fat dairy products. His diet can be even less than 10 percent fat, Ornish said.

Ornish bases his recommendations on several studies that show heart disease has been reversed much more successfully in patients who radically reduce their fat and cholesterol intake. These same studies indicate that heart disease can recur when a person's diet is 30 percent fat.

"In every one of these studies, the people who had heart disease got worse" with a 30, or even 26, percent fat diet, he said.

Ornish sees no need to restrict calories as long as 75 percent of them come from complex carbohydrates. Those are found in abundance in breads, cereals, pasta, rice, dry beans and peas.

Ornish was among speakers who detailed "The Ideal Diet" during the two-day Washington conference, "Diet and Health -- Where is America Going," sponsored by the Center for Science in the Public Interest in celebration of its 20th anniversary.

Fat in the diet is linked strongly to the incidence of heart disease, stroke, hypertension, diabetes and some cancers, said Dr. Frank Sacks, an assistant professor of medicine at Harvard University Medical School.

Sacks was slightly more liberal than Ornish in his assessment of the best diet, although he said "meat, poultry, eggs and cheese should not be in the ideal diet." There is room, he said, for fish, even though it contains some cholesterol and animal fat.

The basic foods in an ideal diet, Sacks suggested, are: cereal, fruits and vegetables and non-fat or 1 percent dairy products. Saturated fats -- those that are solid at room temperature, such as shortening and butter, and a couple of oils, such as palm -- should be limited to 2 percent of one's calories, that is, only 3 to 5 grams a day (1 tablespoon of unsalted butter has more than 11 grams of fat), he said.

As for unsaturated fats, Sacks is more liberal than Ornish, saying that diets consisting of 10 to 40 percent fat are "compatible with longevity," in different parts of the world.

But the ideal level of cholesterol is, however, zero, said Sacks. "There is no biological need to eat cholesterol," he explained, because most people's bodies produce what they need.

When it comes to other foods, Ornish allows some salt in hi ideal diet to enhance flavor, and Sacks permits one or two alcoholic beverages a day.

A diet that is only 10 percent fat means some big changes i eating habits.

With a diet that is 37 percent fat, a person who takes in 2,000 calories a day would be consuming 740 fat calories, or 82 grams of fat. (There are 9 calories per gram of fat.) If that same person reduced his intake to the generally recommended 30 percent, he could consume 600 fat calories, or 66 grams, of fat.

To cut back to a 10 percent fat diet, however, would mean the person would consume only 200 of his calories, or 22 grams, in fats. That's about the amount of fat in 2 tablespoons of mayonnaise or butter, 2 ounces of regular potato chips or half a Burger King Whopper with cheese.

Ornish would not speculate on whether this rigid diet would increase longevity. He did, however, say "you will feel better.

"Not everybody needs to be on a 10 percent fat diet," Ornish admitted, "but if they did, we would have a healthier world."